1. Field of the Invention
The present invention relates to a display device and an electronic apparatus having the same, and more particularly to a flat panel type display device in which pixels each including an electro-optic element are disposed in a matrix, and an electronic apparatus having the same.
2. Description of Related Art
In recent years, a flat panel type display device in which pixels (pixel circuits) each including a light emitting element are disposed in a matrix has rapidly spread in the field of display devices for displaying images. A flat panel type display device using a so-called current driven type electro-optic elements each showing an emission luminance which changes depending on a value of a current flowing through a device, for example, an electro luminescence (EL) display device using an organic EL element utilizing such a phenomenon that application of an electric field across an organic thin film causes the organic thin film to emit a light has been developed and advanced in its commercialization.
The organic EL display device has the following characteristics. That is to say, the power consumption is low because the organic EL element can be driven by using an applied voltage of 10 V or less. In addition, as compared with a liquid crystal display device for displaying an image by controlling an intensity of a light from a light source (backlight) in a liquid crystal cell each pixel including the liquid crystal cell, the high visibility for an image is obtained because the organic EL element is a self-light emitting element. Moreover, the weight-lightening and the thinning are readily realized because the organic EL display device does not require an illuminating device such as the backlight essential to the liquid crystal display device. Furthermore, no afterimage in the phase of displaying a moving image occurs because the organic EL display device has a very high response speed of about several microseconds.
The organic EL display device can adopt a passive matrix system and an active matrix system as a drive system thereof similarly to the case of the liquid crystal display device. However, although the display device adopting the passive matrix system has a simple structure, it involves such a problem that it is difficult to realize the large and high-definition display device because a time period for light emission of the electro-optic element decreases due to an increase in number of scanning lines (that is, in number of pixels), and so forth.
For this reason, in recent years, the display device adopting the active matrix system has been actively developed in which a current caused to flow through an electro-optic element is controlled by an active element provided in the same pixel circuit as that of the electro-optic element, for example, an insulated gate field-effect transistor (in general, a thin film transistor (TFT)). It is easy to realize the large and high-definition display device because the electro-optic element continues to emit a light over a time period of one frame in the display device adopting the active matrix system.
Now, it is generally known that current-voltage characteristics (I-V characteristics) of the organic EL element deteriorate with time (deterioration with time). In the pixel circuit using an N-channel TFT as a transistor for current-driving the organic EL element (hereinafter referred to as “a drive transistor”), the organic EL element is connected to a source side of the drive transistor. Therefore, when the I-V characteristics of the organic EL element deteriorate with time, a gate-to-source voltage Vgs of the drive transistor changes follows the deterioration with time, and as a result, an emission luminance of the organic EL element also changes.
This situation will now be concretely described. A source potential of the drive transistor depends on an operating point between the drive transistor concerned and the organic EL element. Also, when the I-V characteristics of the organic EL element deteriorate with time, the operating point between the drive transistor and the organic EL element fluctuates. Thus, even when the same voltage as that before the deterioration with time is applied to a gate of the drive transistor, the source potential of the drive transistor changes. As a result, since the gate-to-source voltage Vgs of the drive transistor changes, the value of the current flowing through the drive transistor concerned changes. This results in that the emission luminance of the organic EL element changes because the value of the current flowing through the organic EL element also changes.
In addition, in the pixel circuit using a polysilicon TFT, in addition to the deterioration with time of the I-V characteristics of the organic EL element, a threshold voltage Vth of the drive transistor, and a mobility μ of a semiconductor thin film having a channel of the drive transistor formed therein (hereinafter referred to as “a mobility of the drive transistor”) μ change with time. Also, the threshold voltage Vth and the mobility μ differs each pixel due to the dispersion in the manufacturing processes (there is the dispersion in the individual transistor characteristics).
When the threshold voltage Vth and the mobility μ of the drive transistor differs each pixel, the dispersion occurs in the value of the current flowing through the drive transistor each pixel. Thus, even when the voltages which are identical to one another among the pixels are applied to the gates of the drive transistors, respectively, the dispersion occurs in the emission luminance of the organic EL elements of the pixels. As a result, the uniformity of the picture is impaired.
Then, even when the I-V characteristics of the organic EL element deteriorate with time, or the threshold voltage Vth and the mobility μ of the drive transistor changes with time, it is necessary to hold the emission luminance of the organic EL element constant without receiving the influences of them. In order to attain this situation, there is adopted such a constitution for giving each of the individual pixel circuits correction functions such as a compensation function for the fluctuation of the characteristics of the organic EL element, a correction for the fluctuation of the threshold voltage Vth of the drive transistor (hereinafter referred to as “a threshold correction”), and a correction for the fluctuation of the mobility μ of the drive transistor (hereinafter referred to as “a mobility correction”). This technique, for example, is described in Japanese Patent Laid-Open No. 2006-215213 (hereinafter referred to as Patent Document 1).